Just how stupid was Ar-Pharazon?

[the phantom]

This is what happenned when Ar-Pharazon confronted Sauron and his empire (from LOTR Appendix A)-

So great was the might and splendour of the Numenoreans that Sauron's own servants deserted him

AP probably thought "Well hey, if I can take Sauron prisoner without a fight why wouldn't I have a shot at his Ainu relatives?" Remember, he had a lot of back up (LOTR Appendix A)-

He prepared the greatest armament that the world had seen...

and then from the Silmarillion-

In that time the fleets of the Numenoreans darkened the sea upon the west of the land, and they were like an archipelago of a thousand isles; their masts were as a forest upon the mountains, and their sails like a brooding cloud...But the fleets of Ar-Pharazon came up out of the deeps of the sea and encompassed Avallone and all the isle of Eressea, and the Eldar mourned, for the light of the setting sun was cut off by the cloud of the Numenoreans.

Ar-Pharazon believing that he could take Valinor doesn't sound so silly when you consider that he had a better shot at it than any other king in history.